Vertebral compression fractures, as illustrated in FIG. 1, represent a generally common spinal injury and may result in prolonged disability. F. Margerl et al: A comprehensive classification of thoracic and lumbar injuries, Eur Spine J 184-201, 1994. These fractures involve collapsing of one or more vertebral bodies 12 in the spine 10. Compression fractures of the spine usually occur in the lower vertebrae of the thoracic spine or the upper vertebra of the lumbar spine. They generally involve fracture of the anterior portion 18 of the affected vertebra 12 (as opposed to the posterior side 16). Spinal compression fractures can result in deformation of the normal alignment or curvature, e.g., lordosis, of vertebral bodies in the affected area of the spine. Spinal compression fractures and/or related spinal deformities can result, for example, from metastatic diseases of the spine, from trauma or can be associated with osteoporosis. Until recently, doctors were limited in how they could treat such compression fractures and related deformities. Pain medications, bed rest, bracing or invasive spinal surgery were the only options available.
More recently, minimally invasive surgical procedures for treating vertebral compression fractures have been developed. These procedures generally involve the use of a cannula or other access tool inserted into the posterior of the effected vertebral body through the pedicles. The most basic of these procedures is vertebroplasty, which literally means fixing the vertebral body, and may be done without first repositioning the bone.
Briefly, a cannula or special bone needle is passed slowly through the soft tissues of the back. X-ray image guidance, along with a small amount of x-ray dye, allows the position of the needle to be seen at all times. A small amount of polymethylmethacrylate (PMMA) or other orthopedic cement is pushed through the needle into the vertebral body. PMMA is a medical grade substance that has been used for many years in a variety of orthopedic procedures. Generally, the cement is mixed with an antibiotic to reduce the risk of infection, and a powder containing barium or tantalum, which allows it to be seen on the X-ray.
Vertebroplasty can be effective in the reduction or elimination of fracture pain, prevention of further collapse, and a return to mobility in patients. However, this procedure may not reposition the fractured bone and therefore may not address the problem of spinal deformity due to the fracture. It generally is not performed except in situations where the kyphosis between adjacent vertebral bodies in the effected area is less than 10 percent. Moreover, this procedure requires high-pressure cement injection using low-viscosity cement, and may lead to cement leaks in 30-80% of procedures, according to recent studies. Truumees, Comparing Kyphoplasty and Vertebroplasty, Advances in Osteoporotic Fracture Management, Vol. 1, No. 4, 2002. In most cases, the cement leakage does no harm. In rare cases, however, polymethymethacrylate or other cement leaks into the spinal canal or the perivertebral venous system and causes pulmonary embolism, resulting in death of the patient. J. S. Jang: Pulmonary Embolism of PMMA after Percutaneous Vertebroplasty, Spine Vol. 27, No. 19, 2002.
More advanced treatments for vertebral compression fractures generally involve two phases: (1) reposition, augmentation or restoration of the original height of the vertebral body and consequent lordotic correction of the spinal curvature; and (2) filling or addition of material to support or strengthen the fractured bone.
One such treatment, balloon kyphoplasty (Kyphon, Inc.), is illustrated in FIGS. 2A-D. A catheter having an expandable balloon tip is inserted through a cannula, sheath or other introducer into a central portion of a fractured vertebral body comprising relatively soft cancellous bone surrounded by fractured cortical bone (FIG. 2A). Kyphoplasty then achieves the reconstruction of the lordosis, or normal curvature, by inflating the balloon, which expands within the vertebral body restoring it to its original height (FIG. 2B). The balloon is removed, leaving a void within the vertebral body, and PMMA or other filler material is then injected through the cannula into the void (FIG. 2C) as described above with respect to vertebroplasty. The cannula is removed and the cement cures to fill or fix the bone (FIG. 2D).
Disadvantages of this procedure include the high cost, the repositioning of the endplates of the vertebral body are lost after the removal of the balloon catheter, and the possible perforation of the vertebral endplates during the procedure. As with vertebroplasty, perhaps the most feared, albeit remote, complications related to kyphoplasty are related to leakage of bone cement. For example, a neurologic deficit may occur through leakage of bone cement into the spinal canal. Such a cement leak may occur through the low resistance veins of the vertebral body or through a crack in the bone which had not been appreciated previously. Other complications include; additional adjacent level vertebral fractures, infection and cement embolization. Cement embolization occurs by a similar mechanism to a cement leak. The cement may be forced into the low resistance venous system and travel to the lungs or brain resulting in a pulmonary embolism or stroke. Additional details regarding balloon kyphoplasty may be found, for example, in U.S. Pat. Nos. 6,423,083, 6,248,110, and 6,235,043 to Riley et al.; Gantis et al., Balloon kyphoplasty for the treatment of pathological vertebral compression fractures, Eur Spine J 14:250-260, 2005; and Lieberman et al., Initial outcome and efficacy of Kyphoplasty in the treatment of painful osteoporotic vertebral compression fractures, Spine 26(14):1631-1638, 2001, each of which is incorporated by reference herein in its entirety.
Another approach for treating vertebral compression fractures is the Optimesh system (Spineology, Inc., Stillwater, Minn.), which provides minimally invasive delivery of a cement or allograft or autograft bone using an expandable mesh graft balloon, or containment device, within the involved vertebral body. The balloon graft remains inside the vertebral body after its inflation, which prevents an intraoperative loss of reposition, such as can occur during a kyphoplasty procedure when the balloon is withdrawn. One drawback of this system, however, is that the mesh implant is not well integrated in the vertebral body. This can lead to relative motion between the implant and vertebral body, and consequently to a postoperative loss of reposition. Additional details regarding this procedure may be found, for example, in published U.S. patent Publication No. 20040073308, which is incorporated by reference herein in its entirety.
Still another procedure used in the treatment of vertebral compression fractures is an inflatable polymer augmentation mass known as a SKy Bone Expander. This device can be expanded up to a pre-designed size and Cubic or Trapezoid configuration in a controlled manner. Like the Kyphon balloon, once optimal vertebra height and void are achieved, the SKy Bone Expander is removed and PMMA cement or other filler is injected into the void. This procedure therefore entails many of the same drawbacks and deficiencies described above with respect to kyphoplasty.
A proposed improved procedure for repositioning and augmenting vertebral body compression fractures is vertebral body stenting, for example as described in Fürderer et al., “Vertebral body stenting”, Orthopäde 31:356-361, 2002; European Patent Application publication number EP1308134A3; and United States patent application publication No. US2003/0088249, each of which is incorporated by reference herein in its entirety. Veterbral body stenting, as depicted for example in FIG. 3, generally involves inserting into a vertebral body a balloon-tipped catheter (e.g., such as a kyphoplasty balloon) surrounded by a stent (e.g., such as those used in angioplasty). After insertion of the balloon and stent, the balloon is inflated, e.g., using fluid pressure, thereby expanding the stent within the vertebral body. After expansion of the stent, the balloon may be deflated and removed, with the stent remaining inside the vertebral body in an expanded state to fill the vertebral body.
There remains a need for implants and related methods for repositioning and augmenting fractured vertebral bodies and other bones.